Velocity Factor of Coax
 

"Jerry" wrote in message
...

"christofire" wrote in message
...

"Jerry" wrote in message
...

"christofire" wrote in message
...
"Jerry" wrote in message
...

"Harry H" wrote in message
...

The Lindenblad has an overhead null that you might find anoying for
some high elevation passes of LEOs.
Are you open to trying to build a DCA (which is an antenna that I
developed)? I make the claim that there is no other hemispheric
coverage antenna design that performs better than a DCA. But, I
sure am open to being corrected.
The Feb 2008 QST contains an article on the DCA antenna design
concept.
It is my claim that a DCA is extreemely forgiving of construction
errors and uses 4 wire dipoles and 50 ohm coax with 5 RFI type
ferrites as "baluns'.

Jerry KD6JDJ
Given the fact I don't subscribe to QST, domicile Australia, would
you have a copy of the article?

HH


Hi HH

It would be my pleasure to disclose any/all the information I have
relating to the DCA antenna design concept. It is simple. It is two
pairs of crossed dipoles. Each pair is spaced 1/4 wave apart and fed
in phase. One pair is physically mounted 90 degrees from the other
pair. All four dipoles are tilted 30 degtrees from vertical. One
pair is fed 90 degrees later than the other pair.
The concept is so simple and straightfoeward that it is probable that
the concept has been developed before I thought of it. But, I have
been unable to find anything published related to this simple "Double
Cross Antenna"
I told my *Internet buddy*, Patrik Tast, in Finland about the concept
and he found it to be exactly what he needed for reception of NOAA
weather satellite signals. Patrik publishes alot of what I send him
related to the antenna. Patrik shows a section of his web page to
describe the DCA to anyone interested. You can find the QST article
in the section Patrik identifies as ANTENNAS on the first page of his
site http://www.poes-weather.com/index.php.

If you have any questions about the DCA concept you are free to
E-mail me, anytime. Or, if you have any facts or data to show where
I am wrong about how well this antenna performs, or know of something
that performs better, please set me straight.

Jerry KD6JDJ


... but surely this is the same as a Lindenblad array? The tilt of the
dipoles was always a parameter in the Lindenblad, so I wonder how your
DCA differs from what N. E. Lindenblad described in the April 1941
edition of 'Communications'.

Chris


Hi Chris

Several, well educated, antenna experts insist that the DCA is actually
a Lindenblad. If you thought the DCA is a Lindenblad, you are not
alone.
The DCA is not a Lindenblad. The array of four dipoles in a
Lindenblad are fed to produce an overhead null. The four dipoles in a
DCA are fed to produce no overhead null. The DCA is a hemispheric
coverage CP antenna. The Lindenblad is not.
Let me know if you have reason to consider the DCA to be the same as a
Lindenblad. I knew nothing about Lindenblad until after recognizing
the DCA concept.

Jerry m KD6JDJ

Jerry


Perhaps it's a rather fine distinction to say an antenna that has the
same physical form as the Lindenblad array is something different because
the elements are driven differently. The original version that he
patented didn't have rod elements at all (see, for example,
http://www.coe.montana.edu/ee/rwolff...B_antennas.pdf
) but it was the configuration of four slanted dipoles around a central
pole that appears to have borne his name since 1941. Henry Jasik's
'Antenna Engineering Handbook' (now by John L. Volakis, Richard C.
Johnson and Henry Jasik, Chapter 29, Page 34) refers to the configuration
as a Lindenblad array, without being specific about the way the dipoles
are driven. However, applying new names to antennas that exploit well
known configurations seems fairly commonplace in the professional field,
particularly in broadcasting.

Of course you can name your antenna as you please, but there might be
some value in mentioning that it is a development of the Lindenblad
array - you'd certainly need to demonstrate awareness of, and distinction
from, the prior art if you were to seek a patent.

Chris


Hi Chris

I wonder if you have any pictures of a Lindenblad and any radiation
plots. I also wonder if an end fire antenna is the same as a broadside
antenna when they look the same from a distance.

Jerry KD6JDJ


You could take a look at www.bbc.co.uk/rd/pubs/reports/1991-15.pdf which on
pages 9 and 10 has some details of a Lindenblad for 2.5 GHz, with patterns,
that was made from semi-rigid coax and brass tube. The aim in that work was
to achieve the best possible axial ratio in order to reject first-order
reflections from the ground and nearby objects. If I remember correctly,
phase rotation was tried but there really weren't enough variables to get
the axial ratio good enough over the whole sphere, so the dipoles were
driven in phase and the hole in the vertical radiation pattern at the bottom
was 'embraced' as a good thing! In this application, if good axial ratio
couldn't be achieved somewhere it was probably better to avoid radiating in
that direction.

Another Lindenblad, but also arrayed vertically in four tiers, was used at
High Hunsley transmitting station for FM radio. The older photos at
http://tx.mb21.co.uk/gallery/high-hunsley.php show it (at the top of the
structure) but they are rather distant. The modern replacements are
basically crossed dipoles in front of reflectors. A significant challenge
in the design of these (big) things is to get the horizontal radiation
pattern to hand over cleanly from one element to the next around the
structure, without lobes or nulls in either the vertical or horizontal
component. In UK Band II broadcasting, the polarisation is usually said to
be 'mixed' rather than intentionally circular. The Alan Dick company
http://www.alandick.com/broadcast_an...roduct_004.htm still offers a
Lindenblad array for Band II. Their 'FMAC' looks interesting!

As to your question, I'm not certain what you mean so perhaps you could
amplify a bit. Certainly if the paths of currents, their relative
amplitudes and their relative phases in time, appear the same from different
directions then the polarisation should be the same in those directions. A
short helix can operate as a broadside and end-fire antenna at the same time
and I know the quadrifilar helix is a popular option for small L-Band
satellite terminals. However, that radio-camera application imposed
stringent demands for axial ratio and, obviously, the requirements for
satisfactory reception of CP signals from satellites can be less demanding
when CP is used simply to avoid loss on account of mismatched linear
polarisations - when the other sense of CP isn't in use at the same
frequency by the same satellite.

Chris

Velocity Factor of Coax
 

"christofire" wrote in message
...

"Jerry" wrote in message
...

"christofire" wrote in message
...

"Jerry" wrote in message
...

"christofire" wrote in message
...
"Jerry" wrote in message
...

"Harry H" wrote in message
...

The Lindenblad has an overhead null that you might find anoying
for some high elevation passes of LEOs.
Are you open to trying to build a DCA (which is an antenna that I
developed)? I make the claim that there is no other hemispheric
coverage antenna design that performs better than a DCA. But, I
sure am open to being corrected.
The Feb 2008 QST contains an article on the DCA antenna design
concept.
It is my claim that a DCA is extreemely forgiving of construction
errors and uses 4 wire dipoles and 50 ohm coax with 5 RFI type
ferrites as "baluns'.

Jerry KD6JDJ
Given the fact I don't subscribe to QST, domicile Australia, would
you have a copy of the article?

HH


Hi HH

It would be my pleasure to disclose any/all the information I have
relating to the DCA antenna design concept. It is simple. It is
two pairs of crossed dipoles. Each pair is spaced 1/4 wave apart
and fed in phase. One pair is physically mounted 90 degrees from the
other pair. All four dipoles are tilted 30 degtrees from vertical.
One pair is fed 90 degrees later than the other pair.
The concept is so simple and straightfoeward that it is probable
that the concept has been developed before I thought of it. But, I
have been unable to find anything published related to this simple
"Double Cross Antenna"
I told my *Internet buddy*, Patrik Tast, in Finland about the
concept and he found it to be exactly what he needed for reception of
NOAA weather satellite signals. Patrik publishes alot of what I
send him related to the antenna. Patrik shows a section of his web
page to describe the DCA to anyone interested. You can find the QST
article in the section Patrik identifies as ANTENNAS on the first
page of his site http://www.poes-weather.com/index.php.

If you have any questions about the DCA concept you are free to
E-mail me, anytime. Or, if you have any facts or data to show where
I am wrong about how well this antenna performs, or know of
something that performs better, please set me straight.

Jerry KD6JDJ


... but surely this is the same as a Lindenblad array? The tilt of
the dipoles was always a parameter in the Lindenblad, so I wonder how
your DCA differs from what N. E. Lindenblad described in the April
1941 edition of 'Communications'.

Chris


Hi Chris

Several, well educated, antenna experts insist that the DCA is
actually a Lindenblad. If you thought the DCA is a Lindenblad, you
are not alone.
The DCA is not a Lindenblad. The array of four dipoles in a
Lindenblad are fed to produce an overhead null. The four dipoles in a
DCA are fed to produce no overhead null. The DCA is a hemispheric
coverage CP antenna. The Lindenblad is not.
Let me know if you have reason to consider the DCA to be the same as a
Lindenblad. I knew nothing about Lindenblad until after recognizing
the DCA concept.

Jerry m KD6JDJ

Jerry


Perhaps it's a rather fine distinction to say an antenna that has the
same physical form as the Lindenblad array is something different
because the elements are driven differently. The original version that
he patented didn't have rod elements at all (see, for example,
http://www.coe.montana.edu/ee/rwolff...B_antennas.pdf
) but it was the configuration of four slanted dipoles around a central
pole that appears to have borne his name since 1941. Henry Jasik's
'Antenna Engineering Handbook' (now by John L. Volakis, Richard C.
Johnson and Henry Jasik, Chapter 29, Page 34) refers to the
configuration as a Lindenblad array, without being specific about the
way the dipoles are driven. However, applying new names to antennas that
exploit well known configurations seems fairly commonplace in the
professional field, particularly in broadcasting.

Of course you can name your antenna as you please, but there might be
some value in mentioning that it is a development of the Lindenblad
array - you'd certainly need to demonstrate awareness of, and
distinction from, the prior art if you were to seek a patent.

Chris


Hi Chris

I wonder if you have any pictures of a Lindenblad and any radiation
plots. I also wonder if an end fire antenna is the same as a broadside
antenna when they look the same from a distance.

Jerry KD6JDJ


You could take a look at www.bbc.co.uk/rd/pubs/reports/1991-15.pdf which
on pages 9 and 10 has some details of a Lindenblad for 2.5 GHz, with
patterns, that was made from semi-rigid coax and brass tube. The aim in
that work was to achieve the best possible axial ratio in order to reject
first-order reflections from the ground and nearby objects. If I remember
correctly, phase rotation was tried but there really weren't enough
variables to get the axial ratio good enough over the whole sphere, so the
dipoles were driven in phase and the hole in the vertical radiation
pattern at the bottom was 'embraced' as a good thing! In this
application, if good axial ratio couldn't be achieved somewhere it was
probably better to avoid radiating in that direction.

Another Lindenblad, but also arrayed vertically in four tiers, was used at
High Hunsley transmitting station for FM radio. The older photos at
http://tx.mb21.co.uk/gallery/high-hunsley.php show it (at the top of the
structure) but they are rather distant. The modern replacements are
basically crossed dipoles in front of reflectors. A significant challenge
in the design of these (big) things is to get the horizontal radiation
pattern to hand over cleanly from one element to the next around the
structure, without lobes or nulls in either the vertical or horizontal
component. In UK Band II broadcasting, the polarisation is usually said
to be 'mixed' rather than intentionally circular. The Alan Dick company
http://www.alandick.com/broadcast_an...roduct_004.htm still offers a
Lindenblad array for Band II. Their 'FMAC' looks interesting!

As to your question, I'm not certain what you mean so perhaps you could
amplify a bit. Certainly if the paths of currents, their relative
amplitudes and their relative phases in time, appear the same from
different directions then the polarisation should be the same in those
directions. A short helix can operate as a broadside and end-fire antenna
at the same time and I know the quadrifilar helix is a popular option for
small L-Band satellite terminals. However, that radio-camera application
imposed stringent demands for axial ratio and, obviously, the requirements
for satisfactory reception of CP signals from satellites can be less
demanding when CP is used simply to avoid loss on account of mismatched
linear polarisations - when the other sense of CP isn't in use at the same
frequency by the same satellite.

Chris


Hi Chris

The Lindenblad antenna is fed to produce a null toward zenith. The
Lindenblad antenna as defined by Brown and Woodward in the mid 1940s for TV
transmission, has an omniazimuth radiation pattern.
The DCA has no zenith null.

If you consider an antenna with an overhead null to be the same as an
antenna with no null to be the same, I have no expectation that you and I
will agree.

The DCA offers little advantage over a Quad Helix when radiation pattern
is considered.
The DCA is slightly more sensitive toward the horizon than the Quad Helix.
..
The bandwidth of a DCA is far wider than a Quad helix.
The DCA is very insensitive to dimensional errors when built by an
amateur. The Quad Helix is extreemely demanding of prescission of
construction.

The original subject of this thread was related to building an antenna for
reception of Low Earth Orbiting satellites. I figured the OP could
appreciate knowing that a DCA will perform better than a Lindenblad and
needs no series matching transformors.

Jerry KD6JDJ

Velocity Factor of Coax
 

On Tue, 10 Feb 2009 16:04:16 GMT, "JB" wrote:

You might want to consider using RG-6/u instead of RG-59. These daze,
the quality of RG-59 coax is rather marginal. I've seen 80% coverage

Beware of the Aluminum foil and shields though. Once it gets wet there is
no stopping the internal corrosion and will generate broadband noise under
power. This includes both RG6 and 9913 and LMR types. They have been
banned from all commercial sites around here.

Amazing. I can see it's possible to create a diode mixer junction
with electrolytic action between the aluminum foil and copper braid.
I've never seen broadband noise from coax cable. Usually the foil is
mylar or polyester coated and the copper is tinned or silver plated to
eliminate any points of contact. Most of the radio sites I've seen
are stuffed LMR-400 and LMR-600. Heliax is better, but also far more
expensive. Incidentally, if you want to generate RF noise and mixing,
there's nothing better than an abraided Heliax outer jacket, with the
copper shield touching the zinc galvanized tower.

If intermod were really a problem with foil shielded coax, then the
common residential CATV distribution system would have a big problem
with intermod and mixes. While the buried coax is semi-rigid with a
solid aluminum shield, the pole to house drops are usually quad
shielded RG-6/u (sometimes with a messenger wire) with foil and braid
shields.

Bring a knife with you and inspect before you buy. Tinned Copper braid is
better, Silver clad rigid is best.

Mylar or polyester coated aluminum foil is more important. Scrape the
insulation and see if it flakes loose from the foil. I've seen this
on very old 9913 and some new LMR-400 of dubious origin.

Slight change of topic.... do the commerical sties that ban foil
shielded coax also ban nickel plated connectors? Just curious.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

Velocity Factor of Coax
 

In article ,
Jeff Liebermann wrote:

Beware of the Aluminum foil and shields though. Once it gets wet there is
no stopping the internal corrosion and will generate broadband noise under
power. This includes both RG6 and 9913 and LMR types. They have been
banned from all commercial sites around here.

Amazing. I can see it's possible to create a diode mixer junction
with electrolytic action between the aluminum foil and copper braid.
I've never seen broadband noise from coax cable. Usually the foil is
mylar or polyester coated and the copper is tinned or silver plated to
eliminate any points of contact. Most of the radio sites I've seen
are stuffed LMR-400 and LMR-600. Heliax is better, but also far more
expensive. Incidentally, if you want to generate RF noise and mixing,
there's nothing better than an abraided Heliax outer jacket, with the
copper shield touching the zinc galvanized tower.

If intermod were really a problem with foil shielded coax, then the
common residential CATV distribution system would have a big problem
with intermod and mixes. While the buried coax is semi-rigid with a
solid aluminum shield, the pole to house drops are usually quad
shielded RG-6/u (sometimes with a messenger wire) with foil and braid
shields.

I don't think those cases (commercial/repeater and CATV) are
comparable.

As I understand it (anecdotal report and discussions with other
repeater owners/builders), the problem with LMR-400 and similar cables
occurs when the cable is used in a duplex application, with RF
transmit power and the incoming receive signal being carried on the
same cable simultaneously.

Under these conditions, it doesn't take very much broadband noise
generation at all (from the cable, connectors, or nearby
metal-on-metal contacts) to cause a problem. The transmitter might be
pumping 50 volts RMS of power into the cable, while the receiver is
trying to pick off a signal of less than a microvolt at a frequency
only 600 kHz away. That's a rather critical environment which really
can't tolerate more than a tiny percentage of noise generation in the
cables, connectors, or duplexer.

I don't think there's any problem with using these sorts of cables in
commercial or amateur service in *simplex* applications... they can
handle transmitting, or receiving, just fine. It's only when you try
to do both, simultaneously, through the same cable. that the noise
generation can become a problem.

The same can be true of some classes of antenna problems. The
repeater system I help maintain developed a serious desensitization
problem, due to internal corrosion/oxidation which occurred in the
antenna after several years up in the weather. I doubt that the
corrosion/oxidation effect would ever have been noticed if the antenna
were in service as a normal (simplex) base antenna - it didn't affect
the transmit SWR or the receive sensitivity at all,

I believe that CATV distribution is a rather easier situation for the
cable to handle, since the signals being carried are all of a fairly
similar and moderate power level. The downstream carrier levels
delivered to the home seem to be around 1 millivolt... that's more
than 60 dB higher than what a repeater's receiver has to be able to
pick out when it's receiving a just-usable signal from the outer edge
of its service area.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Velocity Factor of Coax
 

"Dave Platt" wrote in message
...
In article ,
Jeff Liebermann wrote:

Beware of the Aluminum foil and shields though. Once it gets wet there
is
no stopping the internal corrosion and will generate broadband noise
under
power. This includes both RG6 and 9913 and LMR types. They have been
banned from all commercial sites around here.

Amazing. I can see it's possible to create a diode mixer junction
with electrolytic action between the aluminum foil and copper braid.
I've never seen broadband noise from coax cable. Usually the foil is
mylar or polyester coated and the copper is tinned or silver plated to
eliminate any points of contact. Most of the radio sites I've seen
are stuffed LMR-400 and LMR-600. Heliax is better, but also far more
expensive. Incidentally, if you want to generate RF noise and mixing,
there's nothing better than an abraided Heliax outer jacket, with the
copper shield touching the zinc galvanized tower.

If intermod were really a problem with foil shielded coax, then the
common residential CATV distribution system would have a big problem
with intermod and mixes. While the buried coax is semi-rigid with a
solid aluminum shield, the pole to house drops are usually quad
shielded RG-6/u (sometimes with a messenger wire) with foil and braid
shields.

I don't think those cases (commercial/repeater and CATV) are
comparable.

As I understand it (anecdotal report and discussions with other
repeater owners/builders), the problem with LMR-400 and similar cables
occurs when the cable is used in a duplex application, with RF
transmit power and the incoming receive signal being carried on the
same cable simultaneously.

Under these conditions, it doesn't take very much broadband noise
generation at all (from the cable, connectors, or nearby
metal-on-metal contacts) to cause a problem. The transmitter might be
pumping 50 volts RMS of power into the cable, while the receiver is
trying to pick off a signal of less than a microvolt at a frequency
only 600 kHz away. That's a rather critical environment which really
can't tolerate more than a tiny percentage of noise generation in the
cables, connectors, or duplexer.

I don't think there's any problem with using these sorts of cables in
commercial or amateur service in *simplex* applications... they can
handle transmitting, or receiving, just fine. It's only when you try
to do both, simultaneously, through the same cable. that the noise
generation can become a problem.

The same can be true of some classes of antenna problems. The
repeater system I help maintain developed a serious desensitization
problem, due to internal corrosion/oxidation which occurred in the
antenna after several years up in the weather. I doubt that the
corrosion/oxidation effect would ever have been noticed if the antenna
were in service as a normal (simplex) base antenna - it didn't affect
the transmit SWR or the receive sensitivity at all,

I believe that CATV distribution is a rather easier situation for the
cable to handle, since the signals being carried are all of a fairly
similar and moderate power level. The downstream carrier levels
delivered to the home seem to be around 1 millivolt... that's more
than 60 dB higher than what a repeater's receiver has to be able to
pick out when it's receiving a just-usable signal from the outer edge
of its service area.

You got it. Only hardline is authorized on the tower. Hardline doesn't
have the problem because it doesn't have a braid to flex in the wind. Braid
is less than satisfactory for RF Ground and lightning protection too.
Copper ribbons and sheets are preferred for grounds, but regular large
copper cable can be substituted in some cases. On a repeater system,
corroded or old braided coax hoses your own receiver and others nearby on
the tower or the next building with noise. Especially in the wind. You
won't hear that on simplex because the receiver is muted when you transmit.

I don't know if you have seen CATV hardline with a tank strapped to the
utility pole. That keeps water out by pressurizing the hardline. CATV
systems DO have problems with corrosion all the time. With all that cable
though, the cost to get it up and running is enormous, and they would rather
be complaint driven instead. They also use circulators so that the stuff
people do in their house doesn't feed back into the system and screw all the
other customers up.

The 2m people, most business PLMR and the Cops don't get it so they complain
about jammers so you get irate service calls. But you can't talk to them
because if you talk technical, they cover their ears, scream LALA LALA!!
accuse you of lying, and they won't pay for the cure anyway (remember, if
you have to think about the answer, you're lying). We do use LMR and 9913
for cheap simplex ground level base station installs because it is cheaper
and easier to route. Don't use it around your rotor though. The foil will
tear up.

LEO receiving antennas was Velocity Factor of Coax
 

Jerry wrote:

Hi Chris

The Lindenblad antenna is fed to produce a null toward zenith. The
Lindenblad antenna as defined by Brown and Woodward in the mid 1940s for TV
transmission, has an omniazimuth radiation pattern.
The DCA has no zenith null.

If you consider an antenna with an overhead null to be the same as an
antenna with no null to be the same, I have no expectation that you and I
will agree.

The DCA offers little advantage over a Quad Helix when radiation pattern
is considered.
The DCA is slightly more sensitive toward the horizon than the Quad Helix.
.
The bandwidth of a DCA is far wider than a Quad helix.
The DCA is very insensitive to dimensional errors when built by an
amateur. The Quad Helix is extreemely demanding of prescission of
construction.

The original subject of this thread was related to building an antenna for
reception of Low Earth Orbiting satellites. I figured the OP could
appreciate knowing that a DCA will perform better than a Lindenblad and
needs no series matching transformors.

Jerry KD6JDJ



Just how good does this antenna have to be. It's not like it's being
used as a probe to measure randomly polarized signals, where AR=1 is
really important.

Quad helix antennas have a reputation for being demanding, but that's
where the performance requirements are demanding. Considering that quad
helix antennas are made by the millions for GPS and by the thousands for
WxSat use on boats, they aren't all that picky, because conventional
mass production tolerances are "good enough".

Relax the performance requirements and the helix is no more or less
difficult than a turnstile or Lindenblad or CP patch. Before the advent
of modern modeling tools, *designing* a quad helix was a huge chore,
especially if you didn't want to use a quad hybrid in the feed network,
but wanted to do the "one a bit long, one a bit short" to get the 90
degrees.

But, returning to the original question, why not a turnstile (crossed
dipoles fed 90 degrees out of phase)? For LEO satellites, you don't
really want a hemispherical pattern anyway. You want something with more
gain at the horizon where the slant range is much greater (thousands of
km) than at zenith (where the range is hundreds of km).

And, for that matter CP is probably not worth worrying about. The loss
from a perfect CP to a perfect linear is 3dB. If you're in a situation
where 3dB is going to kill you, you've got other problems to worry about.

Where CP is really, really nice is when you want to kill multipath from
close by reflectors. Or in deep space applications, where you don't
know the linear orientation of the transmitter/receiver (and you also
ARE worried about eke'ing out the last tenth or hundredth of a dB of
performance)


Jim, w6rmk

Velocity Factor of Coax
 

In article ,
JB wrote:

I don't know if you have seen CATV hardline with a tank strapped to the
utility pole. That keeps water out by pressurizing the hardline.

Shameful. Just *imagine* the effect of all that nitrogen leaking out
into the atmosphere! What won't those cable guys try go get away with
next? (grin)

CATV
systems DO have problems with corrosion all the time. With all that cable
though, the cost to get it up and running is enormous, and they would rather
be complaint driven instead. They also use circulators so that the stuff
people do in their house doesn't feed back into the system and screw all the
other customers up.

The 2m people, most business PLMR and the Cops don't get it so they complain
about jammers so you get irate service calls. But you can't talk to them
because if you talk technical, they cover their ears, scream LALA LALA!!
accuse you of lying, and they won't pay for the cure anyway (remember, if
you have to think about the answer, you're lying).

Heh. You haven't met the senior hardware guy (Scott N6NXI) on our
2M/220/440 repeater project. He's the staunchest opponent of using
foil/braid cable on a repeater that you're ever likely to meet, and
insisted on all of our stuff being done to solid commercial-grade
standards (circulators, bandpass filters on the auxiliary radios,
etc.). Most of the cables inside the rack are custom-cut/routed .25"
heliax.

It was a lot of work (and more money) but it has paid off in
reliability. We've got 3 repeaters and their duplexers in a single
rack, right next to one another, and no crosstalk or intermod or
desense problems... and we share the site with a powerful
paging-service transmitter and a bunch of public-service radio gear.

We do use LMR and 9913
for cheap simplex ground level base station installs because it is cheaper
and easier to route. Don't use it around your rotor though. The foil will
tear up.

Good point - thanks - I'll remember to use a braid-cable jumper when I
put up a hexbeam later this year.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Velocity Factor of Coax
 

JB wrote:

I don't know if you have seen CATV hardline with a tank strapped to the
utility pole. That keeps water out by pressurizing the hardline. CATV
systems DO have problems with corrosion all the time. With all that cable
though, the cost to get it up and running is enormous, and they would rather
be complaint driven instead. They also use circulators so that the stuff
people do in their house doesn't feed back into the system and screw all the
other customers up.

Ever open up a cable amp that has had leaky (water type leaky) coax
dripping into it? Yikes, they usually keep working for a while, with all
kinds of electrolyzing going on. It's a pretty sad sight.


- 73 de Mike N3LI -

LEO receiving antennas was Velocity Factor of Coax
 

"Jim Lux" wrote in message
...
Jerry wrote:

Hi Chris

The Lindenblad antenna is fed to produce a null toward zenith. The
Lindenblad antenna as defined by Brown and Woodward in the mid 1940s for
TV transmission, has an omniazimuth radiation pattern.
The DCA has no zenith null.

If you consider an antenna with an overhead null to be the same as an
antenna with no null to be the same, I have no expectation that you and I
will agree.

The DCA offers little advantage over a Quad Helix when radiation
pattern is considered.
The DCA is slightly more sensitive toward the horizon than the Quad
Helix. .
The bandwidth of a DCA is far wider than a Quad helix.
The DCA is very insensitive to dimensional errors when built by an
amateur. The Quad Helix is extreemely demanding of prescission of
construction.

The original subject of this thread was related to building an antenna
for reception of Low Earth Orbiting satellites. I figured the OP could
appreciate knowing that a DCA will perform better than a Lindenblad and
needs no series matching transformors.

Jerry KD6JDJ

Just how good does this antenna have to be. It's not like it's being used
as a probe to measure randomly polarized signals, where AR=1 is really
important.

Quad helix antennas have a reputation for being demanding, but that's
where the performance requirements are demanding. Considering that quad
helix antennas are made by the millions for GPS and by the thousands for
WxSat use on boats, they aren't all that picky, because conventional mass
production tolerances are "good enough".

Relax the performance requirements and the helix is no more or less
difficult than a turnstile or Lindenblad or CP patch. Before the advent
of modern modeling tools, *designing* a quad helix was a huge chore,
especially if you didn't want to use a quad hybrid in the feed network,
but wanted to do the "one a bit long, one a bit short" to get the 90
degrees.

But, returning to the original question, why not a turnstile (crossed
dipoles fed 90 degrees out of phase)? For LEO satellites, you don't
really want a hemispherical pattern anyway. You want something with more
gain at the horizon where the slant range is much greater (thousands of
km) than at zenith (where the range is hundreds of km).

And, for that matter CP is probably not worth worrying about. The loss
from a perfect CP to a perfect linear is 3dB. If you're in a situation
where 3dB is going to kill you, you've got other problems to worry about.

Where CP is really, really nice is when you want to kill multipath from
close by reflectors. Or in deep space applications, where you don't know
the linear orientation of the transmitter/receiver (and you also ARE
worried about eke'ing out the last tenth or hundredth of a dB of
performance)


Jim, w6rmk


Hi Jim

It isnt clear to me that you read Howard Kowall's original post. He
intends to build his own antenna to communicate with low earth orbiting
satellites. I have information that will allow Howard to design and build
his own antenna that performs better than the design he chose (Lindenblad).
Do you disagree that a DCA will perform better than a Lindenblad?

I have read many of your post and recognize that you are a smart guy with
lots of information about antennas. Thats why I wonder why you'd write
something as stupid as " And, for that matter CP is probably not worth
worrying about". You know that 3dB *is* normally something to try to
achieve while building an antenna. The reason you write that the circular
polarization is minimally significant seems to be that you are attempting to
minimize the value of the DCA. I wonder if you have any facts or data,
measured or calculated, to demonstrate that you know of any antenna that
performs better than a DCA for ground based reception from LEOs

I agree with you that a Turnstile is a good antenna for LEO satellite
communication from Earth. But, I also claim that a DCA will perform better
than a Turnstile. Do you disagree?

Can you tell me more about why you wrote "For LEO satellites, you don't
really want a hemispherical pattern anyway. You want something with more
gain at the horizon where the slant range is much greater (thousands of
km) than at zenith (where the range is hundreds of km). That is precisely
what I tried to address in the QST article. That is precisely why the
DCA performs better than all others. Besides, the DCA is relatively easy
to build , unlike the Quad helix.

Again, do you know of any antenna design that performs better than a DCA
for communication with LEOs from earth and doesnt require pointing?

Jerry KD6JDJ

Velocity Factor of Coax
 

The 2m people, most business PLMR and the Cops don't get it so they
complain
about jammers so you get irate service calls. But you can't talk to them
because if you talk technical, they cover their ears, scream LALA LALA!!
accuse you of lying, and they won't pay for the cure anyway (remember, if
you have to think about the answer, you're lying).

Heh. You haven't met the senior hardware guy (Scott N6NXI) on our
2M/220/440 repeater project. He's the staunchest opponent of using
foil/braid cable on a repeater that you're ever likely to meet, and
insisted on all of our stuff being done to solid commercial-grade
standards (circulators, bandpass filters on the auxiliary radios,
etc.). Most of the cables inside the rack are custom-cut/routed .25"
heliax.

It was a lot of work (and more money) but it has paid off in
reliability. We've got 3 repeaters and their duplexers in a single
rack, right next to one another, and no crosstalk or intermod or
desense problems... and we share the site with a powerful
paging-service transmitter and a bunch of public-service radio gear.

Say R56 and it usually covers everything that matters. Check this out. Use
to take care of a bunch of UHF Micors he

http://eecue.com/images_archive/eecu..._Antennas.html
(no i didn't take the picture)